Rare earth-containing alloy sintered magnets containing Nd2Fe14B as the main phase are known as the highest performance magnets among permanent magnets and used, for example, in various motors, such as voice coil motors in hard disk drives and motors for hybrid vehicles, and in consumer electric appliances.
Sintered magnets used in the field of automotive, which are generally required to have heat resistance, are required to have high coercive force for preventing high temperature demagnetization. Such high coercive force is imparted to the magnets of this type by optimum regulation of the structure of the main phase Nd2Fe14B and the R-rich phase (boundary phase) present in the crystal grain boundaries between the main phase grains, and by containing about a few mass percent to 10 mass percent of Dy, which is rarer as natural resources and more expensive than Nd and Pr.
However, in general, the magnetic remanence (Br) and the coercivity (HcJ) are trading-off, so that increase in Dy content in a magnet for improving HcJ results in decrease in Br.
For suppressing decrease in Br while improving HcJ, it is effective to selectively distribute Dy or Tb, which has higher magnetic anisotropy than Nd or Pr, not in the main phase, but in the boundary phase.
Patent Publication 1 discloses that a magnet with high coercivity is obtained by forming a Dy and/or Tb layer by sputtering over a sintered rare earth magnet, followed by heat treatment, to thereby distribute Dy and/or Tb at a high concentration in the boundary phase on the surface of a rare earth magnet.
Patent Publications 2 and 3 disclose that the effect similar to that disclosed in Patent Publication 1 may be achieved by adhering a fluoride or hydride of Dy or Tb to the surface of a sintered rare earth magnet, followed by heat treatment.
Patent Publication 1: JP-2005-175138-A
Patent Publication 2: JP-2006-303433-A
Patent Publication 3: WO 2008/120784